These gassing processes have become increasingly important owing to their higher productivity, their lower energy requirement and their improved working conditions. Their principle is that a stream of carrier gas/hardening gas is forced through or sucked through the sand foundry part which is in a mould. The sand foundry part consists of a mixture comprising a basic material (for example quartz sand, zircon sand, chromite sand) and one or more binding agents which can be hardened by the hardening gas. The carrier gas used is mostly air or nitrogen. The hardening gas which can act either in a reactive or in a catalytic manner initiates the hardening of the binding agent in the sand foundry part. The reactive hardening gas is thereby almost depleted whereas the catalytic hardening gas is hardly used. The hardening of the binding agent is finished in an economically useful time period, and the sand foundry part can then be removed from the mould for further use such as for the casting of the molten metal.
In practice various gassing methods are used. The best known methods are the cold box method (phenolic resin/isocyanate binder using vaporous tertiary amines as hardening gas), the CO.sub.2 -process (water-glass-binder with CO.sub.2 as the hardening gas), the SO.sub.2 -process (polyurethane/peroxide with SO.sub.2 as a hardening gas), the beta-set process (phenolic resin with methyl formate as the hardening gas) and the red-set process (resin binder and sulphuric acid with acetals as the hardening gas).
Most of the gassing processes function so that the carrier gas/hardening gas stream is forced or sucked through the sand foundry part in the mould once in every hardening circuit. Examples of this are in DE patent application (Offenlegungschrift) No. 27 47 109 and in DE patent 25 26 875.
The hardening gas thereby is always used in a large excess to ensure that the hardening reaction occurs throughout the sand foundry part. This is true for reactive and for catalytic hardening gases. As a result the unused reactive hardening gas, or most of the added catalytic hardening gas, as it is unused or substantially unused, can be found in the exhaust gases.
Since, with the exception of CO.sub.2, all hardening gases used in the various gassing processes are hazardous to health and the environment and since after the hardening reaction they are present at a concentration which in accordance with the current air pollution requirements can not be released safely into the environment, there is the necessity to remove these pollutants from the exhaust fumes. Numerous processes and installations are known for this purpose which are described, for example in EP patent 128 974, DE patent 40 07 798, DE patent application (Auslegeschrift) 26 20 303, DE patent application (Offenlegungschrift) 37 42 449, DE patent application (Offenlegungschrift) 26 21 153, and in GB patent 12 69 203. All these solutions have the disadvantage that the required equipment is expensive to acquire and maintain, and thus the advantages of the gassing processes described above are either partially counterbalanced or the disposal still remains a problem.
From FR patent 24 37 894 there is known a gassing process in which the hardening gas, a catalyst (in particular an amine), is not only lead once but several times through the sand foundry part which is in contrast to the previously described processes. For this purpose the hardening gas is fed into a circuit which goes through the sand foundry part and within which it is recycled together with the carrier gas (air) until the sand foundry part is hardened. With this process the sand foundry part is hardened throughout with the concomitant lowering of the quantity of catalyst provided.
The disadvantage of this process is that after each hardening cycle, the circuit has to be decreased by the initial cycle volume, since otherwise it would become inflated. The exhaust air which needs to be removed from the cycle has to be purified before it can be emitted into the environment despite the decrease in the amount of catalyst used.